In the United States and Canada, the only species found is the Virginia opossum, and it is usually simply referred to as an "opossum" but some regional dialects shorten this further to "possum",[3] but should not be confused with the suborder Phalangeriformes, which are arboreal marsupials in the Eastern Hemisphere that are also called "possums" because of their resemblance to the Didelphimorphia.

Contents

Opossums are frequently considered to be "living fossils",[4] and as a result are often used to approximate the ancestral therian condition in comparative studies.[4][5] However, this is inaccurate, the oldest opossum fossils are early Miocene in age (roughly 20 million years old)[6] and the last common ancestor of all living opossums approximately dates to the Oligocene-Miocene boundary (roughly 23 million years ago) and is at most no older than Oligocene in age.[7][8] Many extinct metatherians once considered early opossums, such as Alphadon, Peradectes, Herpetotherium, and Pucadelphys, have since been recognized to have been previously grouped with opossums on the basis of plesiomorphies and are now considered to represent older branches of Metatheria only distantly related to modern opossums.

Opossums were minor components of South American mammal faunas until the late Miocene, when they began to rapidly diversify.[7] Prior to this time the ecological niches presently occupied by opossums were occupied by other groups of metatherians such as paucituberculatans[9] and sparassodonts[8][10][11]
Large opossums like Didelphis show a pattern of gradually increasing in size over geologic time as sparassodont diversity declined.[10][11] Several groups of opossums, including Thylophorops, Thylatheridium, Hyperdidelphys, and sparassocynins developed carnivorous adaptations during the late Miocene-Pliocene, prior to the arrival of carnivorans in South America. Most of these groups with the exception of Lutreolina are now extinct.[12]

The word "opossum" is borrowed from the Powhatan language and was first recorded between 1607 and 1611 by John Smith (as opassom) and William Strachey (as aposoum).[13] Both men encountered the language at the British settlement of Jamestown, Virginia, which Smith helped to found and where Strachey later served as its first secretary.[14] Strachey's notes describe the opossum as a "beast in bigness of a pig and in taste alike," while Smith recorded it "hath an head like a swine ... tail like a rat ... of the bigness of a cat."[14] The Powhatan word ultimately derives from a Proto-Algonquian word (*wa·p-aʔθemwa) meaning "white dog or dog-like beast."[14]

Didelphimorphs are small to medium-sized marsupials that grow to the size of a house cat. They tend to be semi-arborealomnivores, although there are many exceptions. Most members of this order have long snouts, a narrow braincase, and a prominent sagittal crest. The dental formula is: 5.1.3.44.1.3.4 × 2 = 50 teeth. By mammalian standards, this is an unusually full jaw. The incisors are very small, the canines large, and the molars are tricuspid.

As a marsupial, the female opossum has a reproductive system that includes a bifurcated vagina, a divided uterus and a marsupium, which is her pouch.[26] The average estrous cycle of the opossum is about 28 days.[27] Opossums do possess a placenta,[28] but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional.[29] The young are therefore born at a very early stage, although the gestation period is similar to that of many other small marsupials, at only 12 to 14 days.[30] Once born, the offspring must find their way into the marsupium to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys.[31] Female opossums often give birth to very large numbers of young, most of which fail to attach to a teat, although as many as thirteen young can attach,[32] and therefore survive, depending on species. The young are weaned between 70 and 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size, usually only one to two years in the wild and as long as four or more years in captivity. Senescence is rapid.[33]

The species are moderately sexually dimorphic with males usually being slightly larger, much heavier, and having larger canines than females.[32] The largest difference between the opossum and non-marsupial mammals is the bifurcated penis of the male and bifurcated vagina of the female (the source of the term "didelphimorph," from the Greek "didelphys," meaning double-wombed).[34] Opossum spermatozoa exhibit sperm-pairing, forming conjugate pairs in the epididymis. This may ensure that flagella movement can be accurately coordinated for maximal motility. Conjugate pairs dissociate into separate spermatozoa before fertilization.[35]

Opossums are usually solitary and nomadic, staying in one area as long as food and water are easily available. Some families will group together in ready-made burrows or even under houses. Though they will temporarily occupy abandoned burrows, they do not dig or put much effort into building their own. As nocturnal animals, they favor dark, secure areas. These areas may be below ground or above.[36][37]

Juvenile opossum hissing defensively

When threatened or harmed, they will "play possum", mimicking the appearance and smell of a sick or dead animal. This physiological response is involuntary (like fainting), rather than a conscious act. In the case of baby opossums, however, the brain does not always react this way at the appropriate moment, and therefore they often fail to "play dead" when threatened. When an opossum is "playing possum", the animal's lips are drawn back, the teeth are bared, saliva foams around the mouth, the eyes close or half-close, and a foul-smelling fluid is secreted from the anal glands. The stiff, curled form can be prodded, turned over, and even carried away without reaction. The animal will typically regain consciousness after a period of a few minutes to four hours, a process that begins with a slight twitching of the ears.[38]

Some species of opossums have prehensile tails, although dangling by the tail is more common among juveniles. An opossum may also use its tail as a brace and a fifth limb when climbing. The tail is occasionally used as a grip to carry bunches of leaves or bedding materials to the nest.[39] A mother will sometimes carry her young upon her back, where they will cling tightly even when she is climbing or running.

Threatened opossums (especially males) will growl deeply, raising their pitch as the threat becomes more urgent. Males make a clicking "smack" noise out of the side of their mouths as they wander in search of a mate, and females will sometimes repeat the sound in return. When separated or distressed, baby opossums will make a sneezing noise to signal their mother. The mother in return makes the clicking sound and waits for the baby to find her. If threatened, the baby will open its mouth and quietly hiss until the threat is gone.

Opossums eat dead animals, insects, rodents and birds. They also feed on eggs, frogs, plants, fruits and grain. One source notes their need for high amounts of calcium.[40] To fulfill this need, opossums eat the skeletal remains of rodents and roadkill animals. Opossums also eat dog food, cat food and human food waste.

Many large opossums (Didelphini) are immune to the venom of rattlesnakes and pit vipers (Crotalinae) and regularly prey upon these snakes.[41] This adaptation seems to be unique to the Didelphini, as their closest relative the brown four-eyed opossum is not immune to snake venom.[42] Similar adaptations are seen in other small predatory mammals such as mongooses and hedgehogs. Didelphin opossums and crotaline vipers have been suggested to be in an evolutionary arms race. Some authors have suggested that this adaptation originally arose as a defense mechanism, allowing a rare reversal of an evolutionary arms race where the former prey has become the predator,[43] whereas others have suggested it arose as a predatory adaptation given that it also occurs in other predatory mammals and does not occur in opossums that do not regularly eat other vertebrates.[9] The fer-de-lance, one of the most venomous snakes in the New World, may have developed its highly potent venom as a means to prey on or a defense mechanism against large opossums.[43]

Opossums are also notable for their ability to clean themselves of ticks, which they then eat. Some estimates suggest they can eliminate up to 5,000 ticks in a season.[44]

This section needs expansion. You can help by adding to it.(October 2016)

Opossums are found in North, Central, and South America. The Virginia opossum lives in regions as far north as Canada and as far south as Central America, while other types of opossums only inhabit countries south of the United States.[45] The Virginia opossum can often be found in wooded areas, though its habitat may vary widely.[46] Opossums have been moving north in recent years.[47]

In Dominica, Grenada, Trinidad, Saint Lucia and Saint Vincent and the Grenadines the common opossum or manicou is popular and can only be hunted during certain times of the year owing to overhunting.[73] The meat is traditionally prepared by smoking, then stewing. It is light and fine-grained, but the musk glands must be removed as part of preparation. The meat can be used in place of rabbit and chicken in recipes. Historically, hunters in the Caribbean would place a barrel with fresh or rotten fruit to attract opossums that would feed on the fruit or insects.

In northern/central Mexico, opossums are known as "tlacuache" or "tlacuatzin". Their tails are eaten as a folk remedy to improve fertility.[74] In the Yucatán peninsula they are known in the Yucatec Mayan language as "och"[75] and they are not considered part of the regular diet by Mayan people, but still considered edible in times of famine.

^ abcSiebert, Jr., Frank T. (1975). "Resurrecting Virginia Algonquian from the Dead: The Reconstituted and Historical Phonology of Powhatan". In Crawford, James Mack (ed.). Studies in Southeastern Indian Languages. University of Georgia Press.